Not applicable.
Not applicable.
The present invention concerns electrical filters for eliminating transients and distortion in an alternating current (AC) utility grid and more specifically to active filters for such use.
Ideally, a utility grid for providing three phase AC power to factories and offices (i.e., industrial environments) includes three AC power conductors or lines, each line providing a pure sine wave of current and voltage, the sine waves having equal and constant amplitude and frequency, and each separated from the others by 120xc2x0. The utility lines are linked to facility coupling lines at a point of common coupling (PCC) (i.e., a utility-customer connection point) which in turn provide power to facility equipment. As well known in the power industry, all power electronic equipment can potentially act as non-linear loads creating utility line disturbances and distorting utility waveforms by injecting harmonic currents into the utility grid.
To illustrate the effects of distorting currents on a utility power grid, consider FIG. 1 wherein a utility source 10 is shown connected at a point of common coupling (PCC) to a load 12 (e.g., a first utility customer) and other loads (e.g., other utility customers) represented collectively by numeral 14. The lines that link the PCC to loads 12, 14 are referred to herein as coupling lines. The utility source 10 includes a finite internal impedance Ls. Due to impedance Ls, when load 12 draws a non-sinusoidal current from source 10, the waveform through the coupling lines and at the PCC becomes distorted with harmonic coupling line currents that can cause machinery and equipment connected at the other loads 14 to malfunction.
In addition to voltage waveform distortion at the PCC, other problems related to harmonic currents include additional heating and possibly over voltages in utility distribution and transmission equipment, errors in metering and malfunctioning of utility relays, interference with communication and control signals and equipment damage from voltage spikes created by high frequency resonance""s.
Unfortunately, harmonic or non-linear loads comprise an ever increasing portion of the total load for a typical industrial plant. In fact, by 1992, harmonic loads had become such a pervasive problem that the Institute of Electrical and Electronic Engineers (IEEE) recommended stringent harmonics standards, including strict utilities limitations, in a document referred to in the industry as IEEE Standard 519 which has generally been accepted in North America. Standard 519 was written with the general understanding that harmonics should be within a reasonable limit at the PCC and therefore puts limits on individual load and total (i.e., distortion from all loads connected at a PCC) harmonic distortion.
One potential source of utility grid distortion includes power electronics required to modify utility voltages for driving motors. Generally, power electronic systems for receiving and converting utility voltages into AC voltages suitable for driving an AC motor include two converter stages, the first converter stage being a rectifier stage and the second converter stage being an inverter stage. The rectifier stage receives and converts the AC utility voltages to DC voltage and provides the DC voltage across positive and negative DC buses. The inverter stage receives and converts the DC voltage to AC voltages, usually at a different frequency and amplitude than the utility voltages, and provides the converted AC voltages to motor terminals to drive a motor.
One way that has been adopted in many applications to reduce harmonic distortion at the PCC is to position passive filters between harmonic generating loads (e.g., motor drives at an industrial facility) and the PCC. Passive filters typically include inductor and capacitor configurations designed to have a series resonance at the harmonic frequencies to be mitigated. While simple in design, unfortunately such passive filters have a number of shortcomings. First, passive filters are typically bulky and expensive. Second, passive filters cannot adapt to changes in harmonic frequencies caused by shifts in the fundamental AC frequency. Third, passive filters cannot account for variations in the series impedance of the utility grid.
The disadvantages associated with passive filters may be overcome by use of active filters in which a compensating power source is connected directly to the coupling lines to provide a countervailing or compensating current that effectively cancels the distorting harmonic currents. Harmonic currents, like the fundamental line current, are sometimes positive and sometimes negative (i.e., have positive and negative segments). For this reason, in order to eliminate harmonic currents, compensating active filters must be able to operate as both a current sink during positive harmonic segments and as a current source during negative harmonic segments.
To this end many active filters include a pulse width modulating (PWM) inverter controllable to provide current/voltage to, or sink current/voltage from, a line. To sink and provide power, the PWM power source in many active filters comprises a simple power capacitor linked in parallel with a PWM inverter bridge across positive and negative DC buses. The power capacitor is charged by coupling line harmonics whenever current is sinked from the lines and is discharged whenever used to provide current to the lines.
Active filters can generally be grouped into two different categories including pure shunt active filters and hybrid shunt active filters. U.S. Pat. No. 5,063,532 (hereinafter xe2x80x9cthe ""532 patentxe2x80x9d) which issued on Nov. 5, 1991 and is entitled xe2x80x9cActive Filter Devicexe2x80x9d, describes an exemplary pure shunt active filter. The ""532 patent filter senses coupling line currents, identifies harmonic current waveforms in each line current, compares compensating currents to the harmonic waveforms, adjusts pulse width modulating (PWM) firing signals as a function of the difference between the compensating and harmonic currents and controls a PWM inverter via the firing signals. PWM inverter output lines are linked to the three coupling lines to provide the compensating currents directly thereto thereby eliminating or substantially mitigating coupling line harmonics.
U.S. Pat. No. 5,567,994 (hereinafter xe2x80x9cthe ""994 patent) which issued on Oct. 22, 1996 and is entitled xe2x80x9cActive Harmonic Filter With Time Domain Analysisxe2x80x9d describes an exemplary hybrid shunt active filter that, like the pure shunt filter, senses line currents on coupling lines and identifies harmonic current waveforms in each line current. Unlike the pure shunt filters, the hybrid filter does not include a feedback loop that compares the compensating and harmonic currents. Instead, hybrid filters simply generate PWM firing pulses calculated to generate compensating voltages that should cancel the harmonic currents and then applies compensating voltages to the lines via transformers and passive filters.
While each of the pure and hybrid shunt filters have several advantages, each also has several shortcomings. For example, it has been determined through experimentation that the power capacitor employed in the filters may not alone be able to maintain sufficient charge or may become overcharged during the compensating process. To this end, it should be appreciated that the power capacitor cannot cause a desired compensating current on a linked coupling line unless the capacitor charge exceeds the coupling line voltage level. Where harmonic currents are relatively more negative than positive (i.e., provide a negative DC offset), the DC bus capacitor charge is quickly drained and the capacitor ceases to operate as a compensating source. Similarly, where harmonic currents are relatively more positive than negative (i.e., provide a positive DC offset), the DC bus capacitor may quickly become excessively charged and, where charge is not limited, may be damaged or destroyed.
As another example, in each of the pure and hybrid shunt cases, it has been determined that processing speed is often to slow to compensate for harmonic currents in essentially real time. To this end, in order to cancel harmonic currents, compensating currents must be equal and opposite to the undesired harmonic currents. Accordingly, harmonic current phases and amplitudes must be accurately determined.
There are a number of methods of identifying harmonic current properties including use of analog filter circuits and digital signal processing. Analog filters have the disadvantages of being extremely sensitive to the values of their components and thus being subject to drift in filter frequency and degradation in performance. Frequency domain digital signal analysis techniques (e.g., Fast Fourier Transform) can be extremely stable but are not presently fast enough to provide accurate real time control necessary for the harmonic current mitigation with the current generation of industrial computers.
The ""994 patent hybrid filter teaches one method for relatively quickly and generally accurately determining harmonic current properties. To this end, the ""994 patent filter samples line currents on each of the three coupling lines N times each signal cycle and uses a lookup table to identify voltages for each line N times every cycle, transforms the three phase currents and voltages to two phase, calculates average real and imaginary powers and other related electrical parameters and uses the electrical parameters to identify an effective fundamental sine wave current which is subtracted from the measured coupling line currents to produce accurate two phase harmonic currents. The two phase harmonic currents are then transformed back into three phase currents and used to generate PWM firing pulses that produce compensating currents.
The ""994 patent filter is relatively fast for two reasons. First, by transforming the currents to two phase and carrying out most calculations using two phase data the number of calculations are substantially reduced. Second, the sampling rate N is selected to be a number related to the processor structure. More specifically, N is a multiple of 2 so that the averaging process can be performed by simply shifting a total (i.e., the sum of powers over a cycle period) to the left. For instance, where N is 256, the averaging process can be performed by shifting the total leftward by 8 places.
While fast, the ""994 patent method cannot be performed instantaneously and therefore PWM adjustments to eliminate harmonics are always performed slightly after the occurrence of a harmonic distortion. Despite reducing harmonic distortions appreciably, the resulting line currents still have some residual distortions due to compensation delay. To this end, see FIG. G which illustrates an exemplary coupling line current that occurs in a system employing a ""994 patent filter.
One other shortcoming regarding the active filter industry generally is that industry members typically concentrate on developing control algorithms and corresponding controllers separately for each new type of filter design. Thus, for instance, one algorithm is developed for a hybrid shunt filter while another algorithm is developed for a pure shunt filter. As in any industry, every new development effort is expensive and implementation and support for each algorithm and controller is expensive.
Thus, it would be advantageous to have a controller and corresponding algorithm that avoids problems associated with filter power capacitor charge, essentially eliminates residual harmonics due to calculation delays and that is useable in each of pure shunt and hybrid shunt active filter systems.
An exemplary embodiment of the invention includes an active filter controller for use with both pure and hybrid shunt filters wherein the controller maintains a minimum DC bus voltage required to generate a compensating current on coupling lines and also extrapolates to estimate an expected feedback current to be compensated so that compensation currents are more accurate and harmonic currents are appreciably reduced.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.